Development and results are presented for a subsurface soil to indoor air chemical vapor intrusion model that includes oxygen-limited biodegradation. The algebraic model incorporates a steady-state subsurface gasoline vapor source, diffusion-dominated soil vapor transport in a homogeneous subsurface soil layer, and mixing within a building enclosure. The soil is divided into a shallow aerobic layer including biodegradation and a deeper anaerobic layer in which biodegradation is neglected. Biodegradation of multiple chemicals is included, with aerobic first-order reaction kinetics estimated from measured data. Oxygen is supplied at the soil surface below the building foundation. Oxygen demand is attributed to a sum of multiple biodegrading chemicals and to baseline respiration of native soil organic matter. The model is solved by iteratively varying the aerobic depth to match oxygen demand to oxygen supply. Model results are calculated for ranges of source concentrations, unsaturated soil characteristics, and building parameters. Results indicate vapor intrusion of petroleum hydrocarbons can be significantly less than indicated by estimates that neglect biodegradation.

Ethanol-blended fuel releases usually stimulate methanogenesis in the subsurface, which could pose an explosion risk if methane accumulates in a confined space above the ground where ignitable conditions exist. Ethanol-derived methane may also increase the vapor intrusion potential of toxic fuel hydrocarbons by stimulating the depletion of oxygen by methanotrophs, and thus inhibiting aerobic biodegradation of hydrocarbon vapors. To assess these processes, a three-dimensional numerical vapor intrusion model was used to simulate the degradation, migration, and intrusion pathway of methane and benzene under different site conditions. Simulations show that methane is unlikely to build up to pose an explosion hazard (5% v/v) if diffusion is the only mass transport mechanism through the deeper vadose zone. However, if methanogenic activity near the source zone is sufficiently high to cause advective gas transport, then the methane indoor concentration may exceed the flammable threshold under simulated conditions. During subsurface migration, methane biodegradation could consume soil oxygen that would otherwise be available to support hydrocarbon degradation, and increase the vapor intrusion potential for benzene. Vapor intrusion would also be exacerbated if methanogenic activity results in sufficiently high pressure to cause advective gas transport in the unsaturated zone. Overall, our simulations show that current approaches to manage the vapor intrusion risk for conventional fuel released might need to be modified when dealing with some high ethanol blend fuel (i.e., E20 up to E95) releases.

Detailed site investigations to assess potential inhalation exposure and risk to human health associated with the migration of petroleum hydrocarbon vapors from the subsurface to indoor air are frequently undertaken at leaking underground storage tank (UST) sites, yet documented occurrences of petroleum vapor intrusion are extremely rare. Additional assessments are largely driven by low screening‐level concentrations derived from vapor transport modeling that does not consider biodegradation. To address this issue, screening criteria were developed from soil‐gas measurements at hundreds of petroleum UST sites spanning a range of environmental conditions, geographic regions, and a 16‐year time period (1995 to 2011). The data were evaluated to define vertical separation (screening) distances from the source, beyond which, the potential for vapor intrusion can be considered negligible. The screening distances were derived explicitly from benzene data using specified soil‐gas screening levels of 30, 50, and 100 µg/m3 and nonparametric Kaplan‐Meier statistics. Results indicate that more than 95% of benzene concentrations in soil gas are ≤30 µg/m3 at any distance above a dissolved‐phase hydrocarbon source. Dissolved‐phase petroleum hydrocarbon sources are therefore unlikely to pose a risk for vapor intrusion unless groundwater (including capillary fringe) comes in contact with a building foundation. For light nonaqueous‐phase liquid (LNAPL) hydrocarbon sources, more than 95% of benzene concentrations in soil gas are ≤30 µg/m3 for vertical screening distances of 13 ft (4 m) or greater. The screening distances derived from this analysis are markedly different from 30 to 100 ft (10 to 30 m) vertical distances commonly found cited in regulatory guidance, even with specific allowances to account for uncertainty in the hydrocarbon source depth or location. Consideration of these screening distances in vapor intrusion guidance would help eliminate unnecessary site characterization at petroleum UST sites and allow more effective and sustainable use of limited resources.

Fuel ethanol releases can stimulate methanogenesis in impacted aquifers, which could pose an explosion risk if methane migrates into enclosed spaces where ignitable conditions exist. To assess this potential risk, a flux chamber was emplaced on a pilot-scale aquifer exposed to continuous release (21 months) of an ethanol solution (10% v:v) that was introduced 22.5 cm below the water table. Despite methane concentrations within the ethanol plume reaching saturated levels (20-23 mg/L), the maximum methane concentration reaching the chamber (21 ppm(v)) was far below the lower explosion limit in air (50,000 ppm(v)). The low concentrations of methane observed in the chamber are attributed to methanotrophic activity, which was highest in the capillary fringe. This was indicated by methane degradation assays in microcosms prepared with soil samples from different depths, as well as by PCR measurements of pmoA, which is a widely used functional gene biomarker for methanotrophs. Simulations with the analytical vapor intrusion model "Biovapor" corroborated the low explosion risk associated with ethanol fuel releases under more generic conditions. Model simulations also indicated that depending on site-specific conditions, methane oxidation in the unsaturated zone could deplete the available oxygen and hinder aerobic benzene biodegradation, thus increasing benzene vapor intrusion potential. Overall, this study shows the importance of methanotrophic activity near the water table to attenuate methane generated from dissolved ethanol plumes and reduce its potential to migrate and accumulate at the surface.

Vapor intrusion is associated with subsurface sources of both chlorinated VOCs and petroleum VOCs; however, petroleum vapor intrusion has been reported to occur under a narrower range of hydrogeologic settings. Research conducted over the last several years including field studies, evaluation of large datasets, and modeling studies, has provided an improved understanding of the differences in vapor intrusion associated with chlorinated VOCs and petroleum VOCs. When oxygen is present in the vadose zone, aerobic biodegradation typically results in rapid attenuation of petroleum VOCs diffusing upwards from deeper sources. At many building sites, wind-driven advection and/or building pressure fluctuations provide sufficient oxygen transport below the foundation to support this aerobic biodegradation. In such cases, there is limited potential for vapor intrusion from dissolved sources of petroleum VOCs unless preferential migration pathways are present. These findings support a framework for the evaluation of vapor intrusion at petroleum hydrocarbon sites that involves simple screening for preferential pathways at sites with sufficient vertical separation between the building and a dissolved source (e.g., 3 m) or a LNAPL source (e.g., 10 m), but a more intensive investigation at sites with petroleum sources in closer proximity to the building.